In an effort to improve the basic asphalt paving process, attempts have been made to utilize foamed asphalt processes in place of conventional asphalt pavement mixing methods.
One foamed asphalt process is described in U.S. Pat. No. 2,917,395 to L. H. Csanyi. This process involves use of steam to produce foamed asphalt, which is then mixed with a substrate.
Another approach was proposed in U.S. Pat. No. 3,423,222 to McConnaughay. The method disclosed in that patent involves heating aggregate in a drum and coating the hot aggregate with a turbulent dispersion or cloud of asphalt.
More recently, a process of making foamed asphalt suitable for mixing with unheated aggregate has been developed. This process involves combining cold water with hot liquid asphalt to produce a foamed asphalt, and is described in detail in Australian Pat. No. 433,003.
The use of cold water and heated asphalt to produce a foamed product has showed considerable promise as an alternative to the conventional hot mix paving process. However, it is essential that the asphalt used have good foaming characteristics in order for the foamed asphalt process to be successful.
Unfortunately, it is not uncommon in the manufacture, storage and handling of asphalt to use a silicone defoamer to prevent froth formation. This silicone additive is very effective as a froth preventer, but it remains in the asphalt, and when a silicone-containing asphalt is subsequently used in a foamed asphalt process, the resulting foam is not easily formed and is of poor quality. Some asphalts simply do not have good foaming characteristics, whether or not a defoamer has been added, and the additive of this invention may be used to improve the foaming characteristics of these asphalts.
Prior attempts to improve the foaming characteristics of asphalt which is used to produce a foamed asphalt paving composition have included addition of foaming agents to the water or asphalt prior to generating the foam. One fairly satisfactory material is powdered sodium stearate, which has been added in amounts of a few tenths of a percent based on weight of asphalt to be foamed. This powdered sodium stearate effectively overcomes silicone defoamer if present and allows formation of a good foamed asphalt. However, the use of powdered material is very difficult in an actual paving operation, presenting handling problems and causing difficulty in obtaining uniform dispersion of the additive. Attempts to dissolve metal stearate in a satisfactory oil carrier have not been very successful, as the resulting solutions thicken such that they are not readily pumpable by an additive metering pump or the like.
Solutions of metal stearates in light hydrocarbon solvents are used in the paint industry, but these solutions are unsatisfactory for use around heated asphalt due to adverse effects of light hydrocarbons on the asphalt properties and because of hydrocarbon emission and safety considerations.